1. Field of the Invention
This invention relates to a fluid-filled power unit mount through which a power unit of a vehicle is mounted on a vehicle body, and more particularly to a power unit mount of the type wherein an elastic support body is securely disposed between inner and outer cylindrical members and formed with a chamber filled with a fluid for lowering vibration transmissibility.
2. Description of the Prior Art
In general, a power unit constituted of an engine and a transmission is supported through power unit mounts (engine mounts) on a vehicle body in order to prevent engine vibration and the like from being transmitted to the vehicle body. Such a power unit mount usually includes an elastic support body formed of rubber or the like and securely put between two attachment members which are respectively connected to the vehicle body and the power unit. This elastic support body absorbs vibration transmitted to the power unit mount. Recently in order to improve the ability to lower vibration transmissibility, a fluid chamber to be filled with a liquid is formed in the elastic support body. Such a device is called a fluid-filled power unit mount.
Such a power unit mount is disclosed, for example, in Japanese Patent Provisional Publication No. 61-65935. In this power unit mount, the elastic support body for supporting the load is interposed between inner and outer cylindrical members. The inner cylindrical member is attached to one of the power unit and the vehicle body, while the outer cylindrical member is attached to the other of them. Thus, this power unit mount is of a so-called inner-and-outer cylinder type. Additionally, the elastic support body of this power unit mount is formed with a pair of fluid chambers which communicate with each other through an orifice passage and are filled with liquid. Accordingly, when the elastic support body deforms in accordance with vibration input, the liquid within the fluid chambers move through the orifice passage. By tuning the flowing condition of the liquid at the time of elastic support body deformation, the ability to lower vibration transmissibility under the action of the power unit mount is improved.
With such an inner-and-outer cylinder type power unit mount, the inner and outer cylinders are prevented from being separated from each other even when the elastic support body is broken and the power unit mount can be small-sized. However, difficulties have been encountered in the inner-and-outer cylinder type power unit mount, in which the frequencies of vibration to be damped are unavoidably set within a high vibration frequency region which is far from a low frequency region of so-called engine shake and idling vibration for the following reasons. The fluid chambers are formed in the elastic support body which has a high spring constant, the side walls defining the fluid chamber are thick, and the orifice passage is linear so as to be smaller both in length and cross-sectional area.
In view of the above difficulties, an inner-and-outer cylinder type fluid-filled power unit mount has been as disclosed in Japanese Patent Provisional Publication No. 62-224746 in which one of two fluid chambers is defined by an elastic diaphragm having a smaller spring constant and the orifice passage is arcuate along the inner periphery of the outer cylindrical member to prolong the length of the orifice passage, thus making it possible to tune the mount so as to damp vibration within the low frequency region. However, this power unit mount has only one orifice passage connecting the two fluid chambers, and therefore vibration in only one frequency range within the low frequency region can be effectively damped. Accordingly, it is impossible to damp both engine shake and idling vibration which are within different frequency ranges.